The present invention relates to dicalcium phosphate dihydrate compositions having improved monofluorophosphate compatibility, and to a process for the preparation thereof.
Dicalcium phosphate dihydrate has been used as a dental abrasive agent in toothpastes and powders for many years.
This material is typically produced by first reacting a slaked lime slurry with phosphoric acid to form a dicalcium phosphate dihydrate precipitate, and then separating the dicalcium phosphate dihydrate precipitate from the mother liquor after which it is dried and milled to form the final product as a fine powder.
One serious problem which was initially encountered in the use of dicalcium phosphate dihydrate in toothpaste was the tendency of the dicalcium phosphate dihydrate to "set-up" and become lumpy. When this occurs in toothpaste formulations, it makes it difficult to extrude the toothpaste from the tube in which it is usually packaged.
A second problem was encountered with the advent of the use of monofluorophosphate additives in toothpaste formulations. It was found that the monofluorophosphate components would react with the dicalcium phosphate whereby the monofluorophosphate component was converted from a water-soluble form to an insoluble form. Since the beneficial effect of monofluorophosphate additives in toothpaste are understood to be derived principally from the water-soluble form, it has become important to develop toothpaste formulations which permit an effective amount of monofluorophosphate component to remain in the water soluble state.
The term "monofluorophosphate-compatibility" has been used as a term-of-art to describe the tendency of such formulations to permit the monofluorophosphate component to remain in the water soluble state.
The monofluorophosphate compatibility and stability of dicalcium phosphate dihydrate in toothpaste formulations appear to be related in that the monofluorophosphate compatibility is affected by the stability. Improvements in stability generally result in improvements in monofluorophosphate compatibility.
The monofluorophosphate compatibility of a particular formulation may be determined by a variety of methods. Preferably, the monofluorophosphate compatibility of a formulation may be determined by actually preparing the formulation, placing it in storage for a predetermined period of time under controlled conditions, and then determining the amount of water-soluble monofluorophosphate which remains in the formulation after having been stored under these conditions. Alternatively, a simulated formulation, such as the dicalcium phosphate dihydrate to be tested, glycerine and a known amount of a monofluorophosphate component, such as sodium monofluorophosphate can be "quick aged" by maintaining at an elevated temperature for one or more hours, and the amount of water-soluble monofluorophosphate remaining after such conditioning then determined. There are, of course, many other methods for measuring the relative monofluorophosphate compatibility of various samples of dicalcium phosphate dihydrate.
U.S. Pat. No. 2,287,699 teaches that dicalcium phosphate dihydrate may be stabilized by adding a small amount of an alkali metal pyrophosphate to the mother liquor, at a controlled pH, during the preparation of the dicalcium phosphate. Specifically, it is taught that after precipitation of the dicalcium phosphate in the mother liquor, a small amount of alkali metal pyrophosphate should be added and the entire slurry then heated for a short period of time, while maintaining the pH of the mother liquor above 7.
Alternatively, the precipitate may be treated during the subsequent washing step.
It is also known to those skilled in the art that other forms of pyrophosphate can also be used to stabilize the dicalcium phosphate dihydrate.
Another method for stabilizing dicalcium phosphate dihydrate is disclosed in U.S. Pat. No. 2,018,410. This patent teaches that dicalcium phosphate can be stabilized by the addition thereto of a magnesium salt such as trimagnesium phosphate, magnesium sulfate, magnesium stearate, or dimagnesium phosphate.
Yet another method for stabilizing dicalcium phosphate dihydrate is disclosed in British Pat. No. 1,548,465. This patent teaches a process for stabilizing dicalcium phosphate dihydrate with the use of dimagnesium phosphate trihydrate, which comprises precipitating jointly with the dicalcium phosphate, or precipitating subsequently, directly onto the newly precipitated dicalcium phosphate dihydrate, dimagnesium phosphate trihydrate in an amount of 1 to 50% by weight of dicalcium phosphate dihydrate.
A complete solution to the problems presented by incompatibility between dicalcium phosphate dihydrate and monofluorophosphate additives in toothpastes however, has not yet been found, and the need for further improvements is well-recognized by the industry.